The combustion of fuels containing hydrogen and carbon, such as coal and natural gas, produces significant volumes of gaseous exhaust waste streams that contain one or more undesirable gaseous compounds such as one or more of the acid gases. Acid gases such as carbon dioxide (CO2), sulfur gases (e.g. SO2, H2S), and oxides of nitrogen (NOx), can cause significant environmental pollution and health risks. There has been increasing concern about the damage caused by these contaminants, which has led to an increased demand to reduce their emission, including CO2.
Separation of acid gases, such as CO2 and H2S from gas streams can be achieved via chemical absorption or chemical/physical absorption processes. The most widely used process for CO2 separation and capture from acid gas-containing streams is the chemical absorption process utilizing liquid amine solutions. Aqueous solutions of monoethanolamine (MEA) or diethanolamine (DEA) are commonly used in the wet chemical absorption and low-pressure stripping of CO2. In this process, the CO2 reacts with the liquid amine solution to form a carbamate species. Upon heating, the carbamate species decomposes to release the absorbed CO2 and regenerate the amine solution. This process can be costly and energy intensive. For example, the liquid amine solution has a limited life time due to its degradation through oxidation. Furthermore, the high corrosivity of the utilized amine makes it difficult to use high concentrations of the amine solutions. Typically, only 10-30 wt % solutions of MEA are employed to capture CO2 from the acid gas containing streams, which necessitates the heating and cooling of large volumes of water.
Acid gas capture technology utilizing solid sorbents has increasingly received attention due to its potential for reducing corrosion, energy cost and mass/heat transfer. Such technology uses a porous solid sorbent to reversibly adsorb the CO2 and/or H2S from the acid gas containing streams.
Synthetic zeolite A and X types can be effective adsorbents for CO2. U.S. Pat. No. 3,981,698 to Leppard, U.S. Pat. No. 4,039,620 to Netteland, U.S. Pat. No. 4,711,645 to Kumar, U.S. Pat. No. 4,986,835 to Uno, and U.S. Pat. No. 5,156,657 to Ravi describe the use of 5A, 10A and 13X as CO2 sorbents. In these processes, the molecular sieves physically adsorb the CO2 and are regenerable at ambient temperature and pressure. However, at ambient temperature, desorption cycles are too short to desorb all adsorbed CO2. Consequently, some of the adsorbed CO2 remains on the molecular sieve, which reduces its capacity.
In alternative embodiments of the solid sorbents, amino-containing moieties have been impregnated or grafted onto the surface of the solid supports. For instance, WO 2004/054708 describes the use of a water-tolerant sorbent containing grafted amine on a silica support. U.S. Pat. App. Publ. No 2007/0149398 to Jones et al discloses a method of preparing a CO2 solid sorbent containing a hyperbranched amine polymer covalently bonded to at least one surface oxygen. At atmospheric pressure, the average CO2 capacity was said to be 4.4 mmol (CO2)/g sorbent, or 0.1936 g CO2/g sorbent.
Liquid oxygenated amines, such as diisopropanolamine, have been used in removing CO2 from acid gas containing streams. U.S. Pat. No. 4,044,100 describes the use of liquid mixtures of diisopropanolamine and polyethylene glycol in acid gas removal from gaseous streams. The use of the oxygenated compounds, such as polyols, glycols and ethers in CO2 absorption/desorption cycles of solid sorbents has been examined and used in the space shuffle CO2 removal system.
In U.S. Pat. Nos. 6,364,938; 5,876,488; 5,492,683 and 5,376,614 describe a solid sorbent containing a high surface area solid polymer support and polyethyleneimine/polyethylene glycol. The concentration of both the polyethylenimine and polyol utilized was from 1 wt % to 25 wt % of the total weight of the absorbent. Hicks et al, Journal of American Chemical Society 130:2902 (2008), described a method for impregnating a mesoporous silica support with different amounts of polyethyleneimine. However, the resulting material was very sticky due the addition of the polyethyleneimine to the internal and external surface of the silica support. In addition, a significant pressure drop was observed because of the clogging in the absorbing column. Therefore, the CO2 capacity could not be determined.
U.S. Pat. No. 7,795,175 to Olah describes a process for CO2 absorption using solid nano-particles of a silica support, polyethylenimine and polyol. The concentration of the utilized polyethylenimine was 25 wt % to 75 wt % from the total weight of the sorbent and polyol was in an amount up to 25 wt % of the total weight of the absorbent. The CO2 capacity was 0.117 g (CO2)/g of absorbent. Nevertheless, the small particle size of the sorbent makes it difficult to conduct the absorption/desorption cycles at high pressure due to the pressure drop in the absorption column.
As noted above, the prior art sorbents and methods have one or more undesirable characteristics.